What the hell is going on with search ranking?

I’m observe very strange behaviour of google play ranking algorithm. My app was released about 2.5 months ago, and it have a lot of competitors. But it continues to move up in category top(for tablets). Not so fast, in average few position in day. The number of active users increases every day, the number of downloads per day also have positive trends. Rating is about 4.5 stars. However, results from search sometimes confuse me. I’m using AppAnnie to monitor keywords for USA and my tablet to check position in my country. Let’s say, I have keyword “a”(in english), and for this keyword my app on 80th place. But sometimes google kick me off from search results or move to the end of app list. Of course in this case the number of downloads drops. But a few days later, my app returns approximately to the same position. In same time, my app have translated description and title to my mother language. And if I using keyword “b”(in my mother language) my app on 5th place. Google also moves my app down for few days, but I’m alwasy in top 10. This can happen few times per week.

So, my question is why google “plays” with my app position? It makes hard to get more downloads and get better position.
Does anyone else have the same problem?